FCP: Rename "ObjectChangedNotification" to "ObjectChangedEvent"

The whole Notification stuff was advertised as "event-notifications"
during development a lot. To make it easy for developers to get used it,
"Notification" should be renamed to Event everywhere. Unfortunately,
this is a lot of work with regards to the class hierarchy. Still, we can
rename the stuff to "Event" in FCP message at least, as that is what the
client developers will get in contact with. Also, FCP messages would be
difficult to rename afterwards because it would break client parsers, so
its better to do this now before the code has been released.
FCPInterface.handleSubscribe(): Fix wrong JavaDoc with minimal effort

The backend of this function was recently changed a lot: The whole
mechanism of subscription synchronization was revamped as part of
split-up-subscription-synchronization.
This commit removes the incorrect obsolete JavaDoc and replaces it with
an updated version. The updated version is intentionally left short
though, because we want to get out a new release as soon as possible and
thus I don't want to spend too much time on documentation.
A TODO is added to improve the JavaDoc.

If you need to know how the whole mechanism of event Subscriptions
works, you can read FCPClientReferenceImplementation for a reference
user of it.
FCPInterface: Complete branch split-up-subscription-synchronization

- By removing the FIXME which demanded this branch, it is complete :)
- This resolves the issue
https://bugs.freenetproject.org/view.php?id=6278
- Notice: The whole constant which the FIXME was at is removed
since the branch implements sending of the synchronization as sending a
series of small regular Notifications. For those, the constant
SUBSCRIPTION_NOTIFICATION_TIMEOUT_MINUTES (= 1) right below in the code
is used. So the removed constant was unused in this branch.